In the invention, by the term “excursion” is meant a movement or such operations of air/fuel ratio outward and back from a mean value thereof along a time axis. By the term “rich” is meant an air/fuel ratio smaller than the stoichiometric air/fuel ratio of the fuel in question, while by the term “lean” is meant an air/fuel ratio larger than the stoichiometric air/fuel ratio of the fuel in question. For normal automobile gasoline, the stoichiometric air/fuel ratio is approximately 14.5. Further, the term “catalyst” includes a catalyst itself as well as a catalyst structure which contains the catalyst and works to remove NOx during rich/lean combustion of fuel.
Accordingly, by the term “supplying fuel with periodic rich/lean excursions” is especially meant that fuel is supplied, injected or jetted to a combustion chamber of a diesel engine or a gasoline engine, and is subjected to combustion mainly under the lean conditions (wherein the oxygen concentration in the exhaust gases after combustion of fuel is typically in a range of approximately 5% to 10%) while air/fuel ratio is so adjusted that the combustion atmosphere of fuel is periodically oscillated between the rich conditions and lean conditions. Therefore, “the rich/lean excursions” has the same meaning as “the rich/lean conditions”.
NOx contained in exhaust gases has conventionally been removed by, for example, a method in which the NOx is oxidized and then absorbed in an alkaline solution or a method in which the NOx is reduced to nitrogen by using a reducing agent such as ammonia, hydrogen, carbon monoxide or hydrocarbons. However, these conventional methods have their own disadvantages.
That is, the former method requires a means for handling the resulting alkaline waste liquid to prevent environmental pollution. The latter method, for example, when it uses ammonia as a reducing agent, involves the problem that ammonia reacts with SOx in the exhaust gases to form salts, resulting in a deterioration in catalytic activity at low temperatures. On the other hand, when hydrogen, carbon monoxide or hydrocarbons is used as a reducing agent, the reducing agent reacts preferentially with oxygen since the waste gas contains oxygen in a higher concentration than NOx. This means that substantial reduction of NOx needs a large quantity of a reducing agent.
It was therefore proposed to catalytically decompose NOx in the absence of a reducing agent. However, the catalysts conventionally known for direct decomposition of NOx have not yet been put to practical use due to their low decomposition activity. On the other hand, a variety of zeolites were proposed as a catalyst for the catalytic reduction of NOx using a hydrocarbon or an oxygen-containing organic compound as a reducing agent. In particular, Cu-ion exchanged ZSM-5 or H type (hydrogen type or acid type) zeolite ZSM-5 (SiO2/Al2O3 molar ratio=30 to 40) is regarded as optimal. However, it was found that even the H type zeolite does not have sufficient reduction activity, and particularly the zeolite catalyst was rapidly deactivated on account of dealumination of the zeolite structure when water was contained in the exhaust gas.
Under these circumstances, it has been demanded to develop a more active catalyst for the catalytic reduction of NOx. Accordingly, a catalyst composed of an inorganic oxide carrier material having silver or silver oxide supported thereon has recently been proposed, as described in EP-A1-526099 or EP-A1-679427. It has been found that the catalyst has a high activity for oxidation, but has a low activity for selective reduction of NOx, so that the catalyst has a low conversion rate of NOx to nitrogen. In addition, the catalyst involves the problem that it is rapidly deactivated in the presence of SOx. The catalyst catalyzes the selective reduction of NOx with hydrocarbons under full lean conditions, but it has a lower NOx conversion and a narrower working temperature window (temperature range) than a known three way catalyst. This makes it difficult for such lean NOx catalysts to be practically used. Thus, there has been an urgent demand for a more heat-resistant and active catalyst for the catalytic reduction of NOx.
In order to overcome the above-mentioned problems, a NOx storage-reduction system has recently been proposed as one of the most promising methods, as described in WO 93/7363 or WO 93/8383. In the proposed system, fuel is periodically spiked for a short moment to a combustion chamber in excess of the stoichiometric amount. Vehicles with lean burn engines can be driven at lower fuel consumption rates than conventional vehicles. It is because such vehicles can be driven at a much lower fuel/air ratio than the conventional vehicles. This NOx storage-reduction system for lean burn engines reduces NOx in two periodic steps at intervals of one to two minutes.
That is, in the first step, NO is oxidized to NO2 on a platinum or rhodium catalyst under (normal) lean conditions, and the NO2 is absorbed in an absorbent comprising such an alkali compound as potassium carbonate or barium carbonate. Subsequently, rich conditions are formed for the second step, and the rich conditions are maintained for several seconds. Under the rich conditions, the absorbed (or stored) NO2 is emitted from the absorbent and efficiently reduced to nitrogen with hydrocarbons, carbon monoxide or hydrogen on the platinum or rhodium catalyst. This NOx storage-reduction system works well over a long period in the absence of SOx. However, in the presence of SOx, the catalytic system deteriorates drastically due to the irreversible absorption of SOx at NO2 absorption sites on the alkali compound under either the lean or the rich conditions.
Then, as another method for overcoming the above-mentioned problems, a method, in which although fuel is periodically supplied for a short moment to a combustion chamber in excess of the stoichiometric amount in the same manner as the NOx storage-reduction system, NOx is not absorbed in alkali compounds under lean conditions but decomposed to nitrogen and oxygen on rhodium and/or palladium metal supported on A type zeolite without a reducing agent, and the resulting oxygen is removed by a reducing agent under rich conditions, has been proposed in Japanese Patent Application Laid-Open No. 2001-149758. This method has originally high SOx durability for the reason that NOx does not need to be absorbed in alkali compounds under lean conditions. However, this method involves the problem that it is inferior in NOx purification efficiency to the NOx storage-reduction system.
It is an object of the invention to provide a catalyst and a method for catalytic direct decomposition of NOx contained in exhaust gases wherein fuel is supplied and subjected to combustion with a periodic rich/lean excursions, whereby NOx is generated in the exhaust gases, with high durability in a wide temperature range even in the presence of oxygen, sulfur oxides or water.